1. Field of the Invention
This invention relates to improved membranes for gas and vapor permeation. More particularly, it relates to improved composite membranes and preparation thereof.
2. Description of the Prior Art
Asymmetric and composite membranes capable of selectively separating one component of a gaseous or vapor mixture from another component are well known in the art. For practical commercial operations, such membranes must be durable and essentially free of imperfections, and must be capable of achieving an acceptable level of selectivity of the desired component, exhibiting a high permeation rate for the more rapidly permeable component of the gas or vapor mixture being processed.
Integrally skinned asymmetric membranes are currently used for numerous gas separation processes. Though manufacturing of essentially defect-free, ultra high flux asymmetric membranes is known in the art, for example, in the Pinnau et al. patent, U.S. Pat. No. 4,902,422, and the Sanders et al. patent, U.S. Pat. No. 4,772,392, it is known to be excessively difficult to produce such membranes. Thus, it is common practice in the art to subject asymmetric gas separation membranes to treatments that effectively eliminate defects that may be present in the ultrathin membrane separation layers. The Henis et al. patent, U.S. Pat. No. 4,230,463, addresses the problem of the presence of defects in asymmetric gas separation membranes and discloses the formation of multi-component membranes by applying a coating, typically a silicone rubber coating, to the surface of an asymmetric membrane made of a glassy polymer. Other defect-repair methods are disclosed in the Frisen et al. patent, U.S. Pat. No. 4,877,528, the Peinemann et al. patent, U.S. Pat. No. 4,746,333, and the Smith et al. patent, U.S. Pat. No. 4,776,436.
A different class of gas or vapor separation membranes are composite membranes that are produced by depositing a thin gas separation layer on a porous support. The material of the deposited layer, unlike that of the coating of the Henis et al. patent, determines the gas separation characteristics of the overall membrane structure. Such composite membranes are sometimes more advantageous than asymmetric membranes since they allow decoupling of the separation requirements from the engineering design requirements of the porous substrate. A variety of separation layer materials, support structures and membrane manufacturing methods are known in the art for producing composite membranes. Typical examples of composite gas separation membranes can be found in the Riley et al. patent, U.S. Pat. No. 4,243,701, the Browall patent, U.S. Pat. No. 3,980,456, and the Bikson et al. patent, U.S. Pat. No. 4,881,954. In such composite membranes, the separation layer may comprise a single membrane material or a blend of several materials.
For use in practical commercial operations, composite membranes must have the qualities referred to above, i.e., durability, freedom from imperfections, and an advantageous combination of selectivity and permeability characteristics. Often, however, the gas separation layer deposited on the surface of the porous support, i.e., substrate, does not fully meet the required needs. For instance, it may not be adequately resistant to the solvent effects of the gas/vapor mixture that can condense onto the membrane surface during the gas separation process. Furthermore, the separation layer may contain microscopic residual pores, pinholes or other defects i.e. imperfections that can preclude the attainment of the desired separation characteristics of the composite membrane. Such defects may exist for a variety of reasons, such as the presence of minute impurities in the membrane material, or as a result of damage to the thin separation layer during processing and handling. Thus, difficulties are sometimes encountered in the preparation of defect-free composite membranes for gas separation, particularly when using gas separation materials having very high cohesive energy density, such as polymeric materials that contain ionic groups. As a result, continued efforts are being expended to improve the structure and performance characteristics of composite membranes.
The above-indicated Browall patent discloses a process for patching breaches in a composite membrane in which a layer of sealing polymer material is placed over the surface of the composite membrane to cover particles embedded in the separation layer and to seal pinholes. The Bikson et al. patent, U.S. Pat. No. 4,767,422, discloses a method for repairing defects in composite membranes by post-treating with a volatile solvent, with or without the addition of minute amounts of additives, followed by evaporation of said solvent.
Multilayer composite gas separation membranes are also known in the art that are comprised of a porous support structure and a superimposed intermediate thin layer of high permeability material, with a thin layer of high selectivity, i.e., high separation factor, material further superimposed over the intermediate layer. The intermediate layer serves as a so-called gutter layer to facilitate the permeability of the membrane. It may also further seal some imperfections existing in the exterior gas separation layer. Such multilayer composite membranes of the prior art are complex in construction and require multiple processing steps in the preparation thereof, wherein each membrane layer is formed sequentially in separate film casting or coating steps.
The Cabasso et al. patent, U.S. Pat. No. 4,603,922, discloses the preparation of improved composite membranes of the gutter layer type. A thin layer of amino organofunctional polysiloxane is deposited on the surface of a highly porous polymer substrate, such as a polysulfone substrate, and the amino siloxane units are crosslinked with diisocyanate, with the resulting crosslinked polysiloxane being utilized as a gutter layer. A gas separation layer is coated on the gutter layer to provide a double-layer composite membrane that has a higher separation factor than the crosslinked polysiloxanes. Such an approach, while serving to enhance the permeability characteristics of the membrane and possibly serving to overcome the problems associated with the presence of imperfections in the thin, outer separation layer of such a double-layer composite membrane, nevertheless will be seen to be complex and costly to produce.
Surface active agents have been blended into coating compositions to improve adhesion of the coating layer to the substrate material and to improve the wettability of the substrate by the coating solution. For this purpose, very large quantities of said surface active agents have been employed on occasion. European Patent No. 92/04987 of Hoechst Celanse discloses the use of silicone surfactant for this purpose.
There remains in the art, therefore, a desire for further improvement in the preparation of composite membranes. In particular, there is a desire and genuine need for improved composite membranes capable of overcoming the problem of imperfections in the thin separation layer thereof, without the processing complexity and costs associated with the use of the double-layer composite membranes discussed above.
It is an object of the invention, therefore, to provide an improved composite membrane for gas and vapor permeation application.
It is another object of the invention to provide an improved process for the production of composite membranes overcoming the problems of imperfections in the separation layer thereof.
It is a further object of the invention to provide a multilayer composite gas separation membrane wherein the exterior layer more fully protects the inner gas separation layer from excessive damage during processing and handling.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being pointed out in the appended claims.